Dumb cane (Dieffenbachia picta (Lodd.) Schott ‘Camilla’), family Araceae, is a popular houseplant in Taiwan. During the winter of 2012, dumb canes with dark brown concentric spots on leaves and bright yellow borders were found in a protected ornamental nursery in Wandan township, Pingtung County, Taiwan. On diseased leaves, fungal fruiting bodies were sometimes observed in the concentric lesions and a fungal isolate was consistently isolated from the lesions. A single spore isolate, myr 2-2, was maintained on potato dextrose agar (PDA) for further tests. To fulfill Koch's postulates, the spores of myr 2-2 were suspended in sterilized distilled water containing 0.05% of Tween 20, 1 × 105 conidia ml–1, and then sprayed on leaves of D. picta ‘Camilla’ growing in polypropylene plant pots (about 7 cm in diameter), three plants per treatment. For the control, three plants were sprayed with sterilized distilled water containing 0.05% of Tween 20. Both inoculated and non-inoculated plants were covered with plastic bags and incubated in a growth chamber at 26 ± 1°C. Nine to 12 days after inoculation, symptoms described above were observed on inoculated plants whereas the plants in control remained healthy. The same fungus was reisolated from inoculated plants but not from the controls. Furthermore, the fungal pathogen was identified using its physiological, morphological, and molecular characteristics. In the mycelial growth test, the diameter of the fungal colony reaches 58.2 mm on PDA at 25°C after 14 days. The colonies were floccose, white to buff, and sporulate in concentric zones with olivaceous black to black sporodochia bearing viscid masses of conidia. Conidia were narrowly ellipsoid with rounded ends. The average size of 100 conidia was 6.25 ± 0.04 × 1.63 ± 0.02 μm. For molecular identification, the rDNA internal transcribed spacer (ITS) of isolate myr 2-2 was PCR amplified using ITS1 (5′-TCCGTAGGTGAACCTGCGG-3′) and ITS4 (5′- TCCTCCGCTTATTGATATGC-3′) primer pairs (3) and sequenced. The rDNA sequence was deposited in GenBank (KC469695) and showed 100% identity to the Myrothecium roridum isolates BBA 71015 (AJ302001) and BBA 67679 (AJ301995) (4). According to the physiological, morphological (1,2), and molecular characteristics, the fungal isolate was identified as M. roridum Tode ex Fr. To the best of our knowledge, this is the first report of Myrothecium leaf spot caused by M. roridum on D. picta ‘Camilla’ in Taiwan.
References: (1) D. F. Farr and A. Y. Rossman. Fungal Databases, Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved from http://nt.ars-grin.gov/fungaldatabases/, January 31, 2013. (2) M. Tulloch. Mycol. Pap. 130: 1-42, 1972. (3) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al., eds. Academic Press, New York, 1990. (4) Y. X. Zhang et al. Plant Dis. 95:1030, 2011.